Chronic pain conditions are debilitating diseases affecting at least 5-10 percent of the population at some point in their lives. For the patient suffering from a chronic pain disease, disturbance of, or disruption to their daily life is almost inevitable with a greatly increased risk of developing comorbid psychiatric illness such as depression.
Available drug treatments for chronic pain conditions are subject to various limitations. Non-steroidal anti-inflammatory drugs such as ibuprofen and aspirin and opiates such as morphine, can be effective at treating chronic pain with a predominant inflammatory component, but are much less effective against chronic pain disorders associated with nerve damage (neuropathic pain). In addition the pain relief that can be obtained with opiates is often associated with tolerance and dependence, with increased risk of developing undesirable side effects.
Numerous random-controlled trials have shown that drugs capable of modulating monoamine transmission within the CNS such as the tricyclic antidepressant amitriptyline, are effective in the treatment of chronic pain. Whilst it is likely that inhibition of both noradrenaline and serotonin uptake within specific brainstem and forebrain areas is required to obtain analgesic efficacy, it has recently been reported that injection of a specific dopamine reuptake inhibitor into the forebrain can also inhibit the processing of pain (Burkey, A. R. et al.; J. Neurosci. (1999) 19 (10) 4169-4179).
Thus, although some chronic pain conditions are relatively well treated at present, significant unmet needs remain. There is a continued requirement to develop more selective and effective therapies that are better tolerated, for the treatment of patients with chronic pain conditions.